1. Field of the Invention
The present invention relates to a differential type piezoelectric actuator and in particular, to a differential type piezoelectric actuator of a high reliability which can control a very small displacement with a high accuracy and which can easily be mounted.
2. Description on the Prior Art
With a technical innovation nowadays, more and more fine and accurate positioning mechanism is required such as in a magnetic recording apparatus, a scan type tunnel microscope, an interatomic force microscope, and a stepper used in a semiconductor production apparatus. A piezoelectric material has a characteristic to expand and contract when voltage is applied and this displacement amount can be modified by the material, size, and voltage applied. An actuator employing such a piezoelectric material can control a very small displacement with a high accuracy and attracts attention of engineers. Moreover, a piezoelectric actuator has an advantage over an electromagnetic actuator in that no coil is required, the electromagnetic noise is low, and the size can be reduced.
Japanese Patent Publication No. 4-232678 [1] and others disclose a magnetic head positioning mechanism utilizing a differential type piezoelectric element in a magnetic disc apparatus. In this magnetic head positioning mechanism, two piezoelectric elements fixed to a head arm are connected to a support portion, and a pressure generated by expansion and contraction of the piezoelectric elements is transmitted to the support portion, which in turn rotates a tip end of a support spring connected to the support portion. Thus, positioning is controlled.
Moreover, Japanese Patent Publication No. 3-283580 [2] discloses a mechanism to enlarge a displacement generated by two piezoelectric elements, and Japanese Patent No. 2529380 [3] discloses a head positioning mechanism in which two piezoelectric elements are used to move a support spring in a radial direction of a disc.
However, the piezoelectric elements in the mechanism disclosed in document [1] are connected to both ends of movement of the elements. The pressure transmission loss at the connecting points is large. In order to compensate this loss, the piezoelectric element size should be large and the transmission loss generates a displacement hysteresis, which makes it difficult to control positioning with a high accuracy. Moreover, a stress generated by the repeated drive of the piezoelectric elements causes a destruction at the connection boundary, deteriorating reliability. Moreover, since both ends of the two piezoelectric elements are fixed, the size accuracy should be strict and a size adjustment by machining costs a lot.